There is a continuous and growing need within industry for flat and level close-tolerance concrete floors used in a variety of structures such as office buildings, shopping centers, warehouses, and production and/or manufacturing facilities. Most modern production and manufacturing plants include high-precision machinery and equipment which must be set level on a flat surface. A main benefit from achieving close-tolerance floors is that it will allow for easier installation and set-up of the precision machinery and equipment. This allows a facility to reach its intended level of performance capacity sooner and at a higher level of quality. Facility maintenance costs are also likely to be reduced. When changes to the machinery become necessary, reorganization and set-up of the equipment can also be less costly.
For example, high-density warehouse facilities often utilize narrow aisles and high-reach forklifts to reach tall storage racks containing shelving or storage racks for material goods. Any offset error variation from the desired and ideally level floor can correspond to a proportionally larger vertical offset error at the raised forks of high-reach forklifts. Large vertical offset errors at the forklift forks result in an increasingly greater difficulty in maneuvering the forklift machines along the aisles and while reaching for materials and goods at the upper most shelves. Therefore, flatness or levelness errors in the concrete floor become a limiting factor in the practical design of high-density vertical-storage warehouse facilities. Thus the benefit of having easy to produce smooth and accurately level floors in a high-rise warehouse increases the investment value and efficiency of the facility according to a cost per square foot or cost per square meter basis. In locations where land or real estate values are high or available space is at a premium, such costs are an important factor.
In another example, production facilities containing lines of high precision machinery that must be both level and accurately set with respect to one another also significantly benefit from concrete floors that have been placed accurately and economically. The effort required to adjust or otherwise place shims under the supports of the machinery can be reduced or made unnecessary providing that the concrete floor is accurately level and smooth from the start. This can significantly reduce the cost of initially setting up a production line or later making changes or upgrades to equipment as may be necessary. Smooth and accurately level floors may also contribute to reducing overall maintenance costs related to the equipment over the life cycle of the production facility.
Close-tolerance concrete floors are generally known in the concrete construction industry as “super-flat floors” or simply “super flats”. Super-flat floors are typically expensive for building owners to buy and concrete contractors to produce, since such projects usually require specialized equipment and experienced personnel with a thorough working knowledge of the process. Because of the relatively higher cost of the super-flat floors, often only specified areas of a building floor will be made to super-flat specifications, such as within anticipated aisle ways of a given floor plan. When changes for the floor plan are necessary however, the spacing and location of the aisle ways cannot be easily adjusted or moved. This limitation increases renovation costs and possibly reduces the future investment value and long-term usefulness of the facility.
Close-tolerance, super-flat concrete floors are specified, measured and compared in the concrete industry according to concrete floor profile specification variables. One of these variables is for floor flatness “F-F” and another is for floor levelness “F-L”. These two specifications together are generally referred to in the industry as F-numbers. The F-number system offers a repeatable method for measuring floor quality through statistical means known in the art. Concrete floors having F-numbers near or above the range of F-F 80 and F-L 80 are typically regarded as being super-flat concrete floors.
Super-flat concrete floors are much more difficult and expensive to achieve than those conventionally poured. In order to achieve such super-flat floors, construction work site personnel must be highly trained and skilled, and special equipment is often required to place and finish the concrete. Skilled workers using hand tools can perform the task of striking-off wet, uncured concrete to a specified grade with a conventional floor. However, a large number of workers are required to finish the floor. Production speed of the floor is thus relatively slow with such a conventional process. Additionally, as even the best skilled worker continues to use his tools of the trade, over the course of a day, the worker will fatigue and tire as the day goes on. Human endurance has its typical limitations. This factor can also have an adverse effect on the final F-numbers and quality of the floor. Therefore, because many flat surfaces are finished by manual labor, the surfaces are likely to have relatively poor or inconsistent quality with regard to overall levelness and flatness.
In order to achieve super-flat or otherwise high quality concrete floors, the use of a laser-guided or laser-controlled screeding device, such as the patented LASER SCREED™ screeding machine or device, developed by Somero Enterprises, LLC of Houghton, Mich., may be used to initially level and screed the freshly poured concrete. Other devices or machines for smoothing and screeding uncured concrete that use similar structural elements could be used also. The Somero LASER SCREED™ machine or apparatus and method is described in detail in U.S. Pat. Nos. 4,655,633 and 4,930,935, both entitled SCREEDING APPARATUS AND METHOD, which are hereby incorporated herein by reference. Additionally, U.S. Pat. No. 6,227,761, entitled APPARATUS AND METHOD FOR THREE-DIMENSIONAL CONTOURING, which is hereby incorporated herein by reference, discloses a contouring device and apparatus for producing contoured concrete surfaces over non-flat areas. These would be concrete surfaces such as, for example, those found with driveways, parking lots, paved roads, walkways, and other similar non-planar areas. A detailed review of these inventions will not be included herein but may serve as references as to their specific limitations and help to gain an understanding of the benefits of the invention disclosed herein. For the purposes of illustration and disclosure of the invention herein, a Somero LASER SCREED™ screeding machine will be used as the example.
The typical Somero LASER SCREED™ screeding machine used to produce super flat concrete floors is comprised of essentially the same or similar mechanical elements as that of a standard screeding machine. These elements may include a base machine having a power source supporting a rotatable telescopic boom. The telescopic boom supports a screeding assembly or screed head typically consisting of three elements, a plow, rotating auger, and a vibrating member. The support boom is extended outward over the freshly poured concrete and the screed head is then lowered to the desired grade elevation. The laser control system takes over from this point and the boom is steadily retracted to engage and smooth the concrete. As the boom is retracted, the screed head is continuously controlled by the laser-controlled hydraulic system according to a laser reference plane. This produces a generally level and smoothed concrete surface at the desired elevation. When the boom reaches its retracted position, the screed head is raised out of the concrete. The entire machine is then moved laterally to the next adjacent position and the boom is again extended for another smoothing pass. The screed head is then once again lowered into the concrete where the process is repeated until all the concrete has been leveled and smoothed.
It is important to note that the plow, auger, and vibrator that are on the Somero LASER SCREED™ screeding machine are pivotable about a horizontal axis perpendicular to the direction of travel over the concrete, wherein the pivoting motion is controlled by a set of actuators, such as hydraulic cylinders or the like, via a control system. The control system maintains the proper relative orientation of the screed head components relative to the desired concrete surface throughout any variations of concrete forces against the plow, auger, and vibrator, as well as any horizontal inclination or deflection of the telescopic boom or support structure of the machine. This unique capability is disclosed in detail in U.S. Pat. No. 4,930,935, issued to Quenzi et al., and referred to in U.S. Pat. No. 6,227,761, issued to Kieranen et al., both of which are hereby incorporated herein by reference.
An interesting and significant aspect of existing screed head designs is that the vibrating member is typically set at an elevation that is just slightly below the desired finished surface elevation of the concrete during normal screeding operations. In other words, while the rotating auger cuts, fills, and establishes the concrete at the desired grade, the vibrating member that follows is set slightly below grade. Accordingly, as the concrete is freshly leveled by the auger and the surface is subjected to the final action of the vibrating member, the concrete is essentially pressed downward by the working face of the vibrating member. Due to the resiliency of the freshly poured and smoothed concrete, the vibrated material almost immediately and effectively “springs back” or flows upward, returning to the desired elevation set by the auger. This action is continuous along the full length of the vibrating member. The concrete returns to the desired grade in the wake of the action of the vibrating member as it passes over the concrete. This is a proven characteristic in concrete having typical construction slump consistencies and characteristics. Typically, the trailing edge of the vibrating member is adjusted or set to about ⅛th to ¼th of an inch (about 3 mm to 6 mm) below the desired-level of the smoothed concrete.
There exist, however, limitations toward achieving super-flat high quality floors that are a result of the above-described physical aspect. When the screed head is lowered down onto the concrete at the beginning of a smoothing pass, it is typically overlapped onto the previously smoothed concrete of the adjacent and/or previous set of passes. Because the vibrator is set at a height just slightly lower than desired grade, the vibrator creates a depression in the concrete surface roughly equivalent to the length and width of the vibrating member. With typical concrete floors having non-critical F-number specifications, the landing depressions created by the vibrating member can be simply disregarded in the process. On the other hand, the landing depressions can be typically reduced or possibly eliminated through manual secondary operations using hand tools such as by use of a “highway straight edge” or “bump cutter” tools. However, access to the concrete surface can be a limitation. Workers using these tools may be greatly limited during “wide placement” site conditions or high rates of production. Final concrete trowling and finishing operations can also help to “hide” the landing depressions. However, the actual accuracy of the finished concrete floor surface is likely to remain in question. With super-flat concrete floors, however, the created landing depressions become an even greater limitation toward achieving high-quality floors having high F-number characteristics.
The degree of the created “landing depression” is often dependent on a number of factors. An experienced screeding machine operator can reduce the creation of landing depressions by the carefully coordinated practice of lowering the screed head into the concrete while beginning retraction of the boom. The vibrator may be turned off temporarily, and then quickly turned back on again just at the correct moment in time during the landing. This coordinated technique is known by some experienced screeding machine operators as a “soft landing”. However, such soft landings can be difficult to achieve on a consistent or repeatable basis, and are largely dependent on the level of skill and experience of the screeding machine operator. In addition, the slump condition, degree of cure, and other physical characteristics of the uncured concrete can play a large role in the results.
A further factor beyond that of the control and experience of the operator becomes apparent when soft landings are made on concrete that has already begun to set-up or cure. Concrete that has been leveled and smoothed and then left undisturbed for a period of time will progressively begin to loose its resiliency or ability to flow. The length of time is not easily determined and is subject to many variables such as the prevailing conditions that exist at the site or the mix design of the concrete. Warm, dry and windy conditions may cause the concrete to quickly dry and harden at the surface, while cool and damp conditions may have the opposite effect. Concrete mix designs may also exhibit varying degrees of allowable working time before the resiliency or workability of the material is lost. For example, low slump concrete is by definition stiff and less resilient than high slump concrete, while high-slump concrete flows more readily and smoothly than low-slump concrete and is more easily worked. Also, low slump concrete may be more difficult to work, but often offers higher cure strength by containing less water in the mixing ratio. These variables are important factors with respect to the soft landing of the vibrating member of a LASER SCREED™ screeding machine or other screeding machine when producing high-quality super-flat floors.
A typical wide-placement concrete pour, for example, might consist of a set of eight to sixteen screeding passes from left to right before another row is started. This number of consecutive passes would normally complete the full width of a wide-placement concrete pour. By the time the screeding device returns to the beginning of the next series of smoothing passes, the earlier smoothed concrete may have already begun to set-up. In this case, the screed head must overlap onto the earlier smoothed concrete to produce a substantially continuous and uniform surface. This is where soft landings with the screed head become highly important and valuable. For best results, the vibrating element should not be permitted to substantially or fully engage the already setting concrete within the overlap area of the smoothing pass. If contact between the vibrator and the earlier smoothed concrete is made and sustained, there exists a high likelihood that a landing depression or other irregularity will be created in the previously smoothed and already setting concrete. As the screed head continues onto the freshly poured concrete section, the action of the vibrating member may then again be correct under normal conditions. The area of transition between freshly placed concrete and concrete that has already been screeded and begun to set-up is known in the industry as a “cold joint”. Cold joints are usually minimized as much as possible, however the complete elimination of overlap areas is not reasonably practical. Overlapping the screed head onto previously screeded areas is an inherently necessary and accepted part of the process.
Therefore, there is a need in the art for a concrete smoothing and leveling apparatus that is capable of repeatedly and consistently finishing a concrete surface to a close-tolerance or super-flat level of quality. The apparatus should also help to reduce or substantially eliminate manual labor processes and their inherent variations, and should provide less expensive and higher quality concrete floors and surfaces.